Such structural parts are used as blades for a turbodrive mechanism and the series of holes fulfill various functions, the holes being formed by various processes, such as laser drilling or electrochemical drilling. The formation of the holes disturbs the longitudinally extending grain boundaries, particularly if the spacing between the holes is small, since grain-boundary defects and grain-boundary precipitations represent nuclei for formation of corrosion cracks, which weaken the material between the holes due to the migration of the cracks. This can lead to premature failure of the structural part when high mechanical and thermal stresses are applied, and the service life of the turbodrive mechanism can be adversely reduced which leads to need for increased inspection and service time.
These disadvantages are particularly serious when the series of holes is formed by means of laser technology, and particularly for a columnar, crystallized structural part in which the position of the grain boundaries is not predictable. The position of the longitudinally extended grain boundaries is the result of the stochastic nucleation processes on a cold plate, from which the molten metal advances along a solidification front into a ceramic shell mold for the formation of the structural part. Further, the position of the grain boundaries is dependent on the crystallographic orientation of the nuclei with respect to the locally acting temperature gradients, whereby a natural grain selection occurs. Grains with a &lt;001 &gt; orientation parallel to the temperature gradients are preferred with respect to growth kinetics, which leads to the fact that only a few grains grow out from a starter or from a cold plate.
Grain boundaries, which weaken the material, form between the grains, so that grain-boundary strengtheners, such as C, B or Zr are alloyed with the molten metal alloy for such structural parts. These strengtheners preferably precipitate at the grain boundaries during the solidification of the molten material; however, they reduce the melting point therein by up to 100.degree. C.
If such grain boundaries are encountered during laser drilling, then, particularly in the case of thicker walls and longer drilling lengths, this leads to problems, since on the one hand, depositions of laser-vaporized material of up to several hundred micrometers may arise, in which cracks form, and on the other hand, a melting and a cracking of grain boundaries, up to 100 micrometers deep, may occur. The crack formations of the depositions and the grain boundaries may connect together and cause a serious weakening of the structural part, ultimately leading to failure of the turbodrive mechanism. Therefore, laser drilling of structural parts made of materials of columnar structure that are solidified in an oriented manner cannot be carried out in a cost-favorable way, particularly for dynamically highly-stressed blades in a turbodrive mechanism. In this case, one is obliged to use fundamentally more expensive single-crystal materials, whose production is more expensive and is associated with a high rate of rejection.
With thinner wall, the risk of formation of depositions is less in laser drilling, but there is the danger that when closely adjacent holes are formed, even with concurrent cooling to protect the thinner material from excess heating, grain-boundary cracks will be produced during the laser drilling which will weaken the structural part between the holes.